1. Field of the Invention
The present invention relates to phase difference controllers and methods for controlling the phase difference for use in equipments with a photoelastic modulator, such as ellipsometers, optical rotatory dispersion meters (ORD), circular dichroism dispersion meters (CD), linear dichroism dispersion meters (LD), and linear birefringence dispersion meters (LB), etc.
2. Description of Prior Art
FIG. 6 shows a conventional ellipsometer.
Here, continuous spectrum light is radiated from a light source 10, and wavelength selection is carried out for that light by a spectrometer 12. Then, passing through a polarizer 14, the light is converted to linearly polarized light. Furthermore, while the light passes through a photoelastic modulator 16, the modulator produces the phase difference .delta. between the linearly polarized components whose electrical vectors vibrate in the directions perpendicular to each other. This phase difference .delta. varies as .delta.=.delta..sub.0 sin .omega.t responding to the voltage V.sub.0 sin .omega.t applied to the photoelastic modulator 16 from the driving circuit 18. (In more strict expression, .delta. should be described by .delta.=.delta..sub.0 sin (.omega.t-.PHI.). But it is assumed for simplicity that .PHI.=0 because this assumption does not affect the basic discussion of this invention.) Here, .delta..sub.0, .omega., and t are phase difference amplitude, angular frequency, and time respectively. After passing through the photoelastic modulator 16, the light is incident on the sample 20 at an angle .PHI., reflected from sample 20 and travels through analyzer 22. Then, finally, the light is detected by photomultiplier 24.
The dc component of the output of photomultiplier 24 is selectively amplified by the dc amplifier 26 and the resulting amplified output is applied to the sensitivity adjustment circuit 28. The sensitivity adjustment circuit 28 adjusts the sensitivity of the photomultiplier 24 so that the magnitude (voltage) of this dc component is maintained constant.
On the other hand, the ac component of the output of the photomultiplier 24 is applied to the lock-in amplifiers 30 and 32 via capacitor C.sub.1. The driving circuit 18 supplies the reference signals V.sub.r sin .omega.t and V.sub.2r sin 2.omega.t to the lock-in amplifiers 30 and 32 respectively. The lock-in amplifiers 30 and 32 produce, at the output, voltages A.sub.1 and A.sub.2 proportional to the amplitudes of the components of angular frequencies .omega. and 2.omega. included in the input signal, respectively. These voltages, A.sub.1 and A.sub.2, are supplied to the analogue-to-digital converters 34 and 36 to transform them to digital signals. The resulting digital signals are applied to the microcomputer 38.
By setting the phase difference amplitude .delta..sub.0 so that the value of the Bessel function of order zero J.sub.0 (.delta..sub.0) is zero, that is, by setting .delta..sub.0 =2.405 rad, the microcomputer 38 can determine easily the required values to be measured.
Here, if the wavelength of spectrometer 12 is scanned maintaining the amplitude V.sub.0 constant, the phase difference amplitude .delta..sub.0 will change. Therefore, while the microcomputer 38 scans the wavelength of spectrometer 12, it adjusts the amplitudes V.sub.0 of the output voltage of the driving circuit 18 depending on the wavelength .lambda.. The relation between .lambda. and V.sub.0 is beforehand programmed into the microcomputer 38.
Based on .lambda., .delta..sub.0, A.sub.1, and A.sub.2, the microcomputer 38, in the known manner, determines the complex index of refraction of the substrate of sample 20 or the thickness and the complex index of refraction of the film formed on the surface of the substrate.
However, when the temperature of the photoelastic modulator 16 itself or the temperature of the atmosphere changes, the phase difference amplitude .delta..sub.0 changes. As a consequence, measured results are not accurate. Due to the essentially same reasons, similar problems occur in optical rotatory dispersion meters (ORD), circular dichroism dispersion meters (CD), linear dichroism dispersion meters (LD), linear birefringence dispersion meters (LB), etc.